This is my solution, I needed 3 1/2" so 2X4s on edge, the rear section is filled with styrofoam insulation and the front part is for my plywood X supports to steady the camper when it is off the truck.
I have added a step at the rear since I run with the tailgate off. The step is held on with four bolts for easy removal.
Here are the X supports stowed, the narrow plywood is the base for the supports, with the 4 pieces of 3/4" plywood plus strips it is 1/4" above the 2X4s, so that's what takes the weight.
And my X supports... all this will be painted when the snow is gone and the temperature is above freezing!
